In recent years, due to remarkable technical proceedings in solid state image pickup devices for use in cameras such as digital still cameras and video cameras, small charge-coupled devices (“CCD”) and similar devices have been developed. In addition, the small and light optical systems are in demand. In particular, there is a need for smaller and thinner optical systems to be used in cellular telephones and portable information terminals, since these devices are required to be smaller and thinner.
On the other hand, as an optical system used in the conventional cellular telephones and portable information terminals, a single focus lens which has a small and thin configuration is known. The length of the optical system is short and the volume is not so large. For example, this type of conventional optical system is disclosed in Japanese Patent Application Laid-open No. Hei-11-069214 and Japanese Patent Application Laid-open No. 2002-290523.
Further, an optical system changing the direction of incident object light perpendicularly is known by arranging the prism at the most object side in the optical system. For example, this type of conventional optical system is disclosed in Japanese Patent Application Laid-open No. Hei-9-211287.
The demands of the market and the marked advances in IT technology require an optical system (zoom lens) which can take a image not with a single focus lens but with a power variation lens from the short-focal end to the long-focal end, depending on the imaging object. In addition, the optical system can be used in small and thin devices such as cellular telephones and personal digital assistants (PDAs).
However, even in a conventional lens system with a prism, the distance from an incident object to the imaging element is long, since many lenses are employed at the imaging side of the prism, thus no preferable zoom lens has been obtained for such devices as cellular telephones, personal digital assistants, and mobile personal computers, which have a small size and short total length and are easy to fit in a space with small depth and restricted width.
The present invention has been made to solve these problems, and the object of the present invention is to provide a lens system with fewer lenses, a small and thin size and light weight. Furthermore, the lens system of the present invention has high quality optical characteristics and is applicable for devices such as a cellular telephone and mobile information terminal.